A known method of producing a fluoropolymer comprises carrying out emulsion polymerization in the presence of a fluorinated emulsifier.
In this emulsion polymerization, the fluorinated emulsifier produces the following two main effects: (1) increasing a yield of emulsified particles and increasing a rate of polymerization and (2) stabilizing the emulsified particles.
However, those fluorinated emulsifiers in general use have a fluoroalkyl group containing 7 or more carbon atom and, therefore, a problem arises, namely they are difficult to remove from a fluoropolymer obtained. Accordingly, investigations have been made in search of an emulsion polymerization method by which such problem can be solved.
In recent years, compounds having a radical-polymerizable reactive unsaturated bond and a hydrophilic group, namely the so-called reactive emulsifiers, have been proposed as emulsifiers substitutive for fluorinated emulsifiers (cf. e.g. Patent Document 1: Japanese Kokai Publication H08-67795).
Among the proposals so far offered for polymerization using a reactive emulsifier, there are, for example, emulsion polymerization to be carried out in the presence of a fluorinated, vinyl group-containing emulsifier and a chain transfer agent (cf. e.g. Patent Document 2: International Publication WO 05/097835), the method comprising carrying out a first polymerization step using a water-soluble radical polymerization initiator and a second polymerization step using an oil-soluble radical polymerization initiator, both in the presence of a fluorinated, vinyl group-containing emulsifier (cf. e.g. Patent Document 3: International Publication WO 05/097836), and emulsion polymerization to be carried out in the presence of a fluorinated, vinyl group-containing emulsifier and a bromine atom- and/or iodine atom-containing saturated aliphatic compound (cf. e.g. Patent Document 4: International Publication WO 05/097846).
Reactive emulsifiers are copolymerized by the action of the radical-polymerizable reactive unsaturated bond and, therefore, even when they are used in small amounts, a large number of emulsified particles can be obtained and they have an advantage in that they will “not remain as emulsifiers”. However, reactive emulsifiers are consumed and introduced into a fluoropolymer during a polymerization reaction and, if they are added singly in amounts necessary for maintaining a dispersion stability of dispersions produced, they may possibly impair the characteristics of the fluoropolymer.
An emulsion polymerization method to be carried out in the presence of a hydrocarbon-based emulsifier in lieu of a fluorinated emulsifier has also been proposed (cf. e.g. Patent Document 5: International Publication WO 05/063827). This hydrocarbon-based emulsifier can give an increased number of emulsified particles but cannot stabilize an emulsion to a satisfactory extent in some instances.